In listening devices of this kind it is a problem that the microphones need to be closely matched in order for a possible directional computational algorithms to function optimally. In order that the microphones stay matched over a long period, an automatic matching process is introduced. Here the signals from the microphones are continually analysed to ensure that over time there is no big difference in the output level from the microphones. In such listening devices it is also a problem that when the casing is accidentally touched or touched when applied to the ear, very loud sound output levels may be produced as the microphones are very sensitive to noise propagated through the material of the casing walls.
If substantial differences in the input to the microphones should occur, this might corrupt the outcome of the automatic matching process. Further, it has been discovered that such large differences are most likely to coincide with the occurrence of large and unpleasant noises which the user would prefer not to hear, like the noise which is produced when the casing is touched by the user. In hearing aids a large gain or amplification of the audio signal is introduced to compensate for the hearing loss of the user. This amplification amplifies all signals, wanted as well as non-wanted. The wanted signals usually originates some distance from the hearing aid and arrives travelling through the air. Noise from touching the hearing aid is very unpleasant since it results in a loud output signal from the hearing aid because of the frictional resistance, the banging from the acceleration of fingers, etc. The noise increases as the origin of the noise moves closer to one of the microphones in a multi-microphone hearing aid.
To overcome the above problem the invention provides a method for processing the signals from two or more microphones in a listening device whereby the signals from the microphones are analyzed in order to detect when the casing is touched, whereby changes in the signal processing are effected whenever touching of the casing is detected.
According to the invention, the time dependent differences in short term energy in the signals from the microphones is determined. This time related change may in itself provide information as to possible touching of the hearing aid casing, and also it may be further analysed to detect the occurrence of something or somebody touching the hearing aid casing with a higher degree of certainty.
Preferably the time related change in difference in the short term energy content in the microphone signals is used to determine the rate of change in difference between the short term energy of the microphone signals. Here touching noise will change the difference in measured energy between the two microphones very fast compared to other signals received by the microphones. Noise generated by wind will also change the input, but since it originates from turbulence, the mean energy level from the two microphones will not change fast because of the uncorrelated nature of the wind noise. Other ways of detecting touching noise are possible, but the short term energy differences are easily detected and it is easy to determine a reliable threshold where the signal processing should react to compensate for the loud noise. The threshold my comprise both a threshold for the absolute value of the difference in short term energy and a threshold for the rate of change of the difference in short term energy.
In an embodiment of the invention, the changes in the signal processing which are effected whenever touching of the casing is detected comprises short term muting or attenuation of the output signal in order to protect the user from the annoyance of the loud sounds caused by the touching of the casing.
In yet another embodiment of the invention the changes in the signal processing which are effected whenever touching of the casing is detected comprises a time limit shut down of an automatic microphone process. In this way it is ensured that the matching process is not disturbed by the large short term differences in the energy.
In a further embodiment of the invention the changes in the signal processing which are effected whenever touching of the casing is detected comprising lasting changes in the processing of the audio signal presented to the user of the listening device. Such changes could be program shifts, volume up or down or permanent muting of the listening device according to the wishes of user.